Magnetoresistive random access memories (MRAMs) use magnetization switching. Spin transfer torque MRAMs employing a spin transfer torque writing method operate at a high speed, can be highly integrated, and have high durability. Thus, such MRAMs are expected to be general-purpose nonvolatile random access memories.
Magnetic tunnel junction (MTJ) elements are used as storage elements of the spin transfer torque MRAMs. An MTJ element includes a storage layer having a magnetic layer in which the direction of magnetization can be switched by a write operation of the memory, a reference layer having a magnetic layer in which the direction of magnetization is fixed to one direction, and a tunnel barrier layer located between the storage layer and the reference layer to form a tunnel barrier. The electric resistance of the MTJ element switches between a low-resistance state and a high-resistance state when a current flows in a direction perpendicular to the film plane of the MTJ element depending on whether the magnetizations of the storage layer and of the reference layer are parallel or antiparallel to each other. Data (information) can be read from the MTJ element using the difference in resistance between the parallel state and the antiparallel state.
The spin transfer torque writing is performed by causing a current to flow in a direction perpendicular to the film plane of the MTJ element to switch the direction of magnetization of the storage layer. For example, in order to switch the direction of magnetization from the antiparallel state to the parallel state, a current is caused to flow so that electrons flow from the reference layer to the storage layer. The direction of the current is opposite to the direction of the electrons, from the storage layer to the reference layer. As a result, the spin transfer torque acts on the magnetization of the storage layer to turn to the direction parallel to the direction of magnetization of the reference layer. If the current has an intensity of a predetermined threshold value, the direction of magnetization of the storage layer can be switched. On the contrary, in order to switch the direction of magnetization from the parallel state to the antiparallel state, a current is caused to flow so that electrons flow from the storage layer to the reference layer. In this case, the spin transfer torque acts on the magnetization of the storage layer to turn to the direction antiparallel to the direction of magnetization of the reference layer. By changing the direction of electrons in such a manner, data can be rewritten.
A resistive random access memory employing spin transfer torque writing applies an electric current to the MTJ element through a common path in a read operation and a write operation. This leads to potential data-rewriting in read operations, read disturb. In order to avoid the risk of read disturb, a method is used in which a read current flowing through the MTJ element in a read operation is set to be lower than a write current flowing in a write operation. The probability of occurrence of read disturb can be reduced by this technique. However, decreasing the read current would lead to decreasing the read sensitivity. For this reason, the value of the read current that can be practically used has a lower limit.
In order to avoid read disturb, a method is proposed in which the probability of occurrence of read disturb is reduced by setting the pulse width of read current to be narrower than that of write current. The pulse width of write current in a memory required to operate at a high rate, however, should be reduced, and therefore the pulse width of read current should be reduced further. Since the pulse width of read current has a lower limit due to the read sensitivity and a problem of delay in current pulse caused by wiring, it is rather difficult to set the pulse width of the read pulse narrower than that of the write pulse.